Controlling inductive power transfer systems

ABSTRACT

An inductive power transfer system comprises a primary unit, having a primary coil and an electrical drive unit which applies electrical drive signals to the primary coil so as to generate an electromagnetic field. The system also comprises at least one secondary device, separable from the primary unit and having a secondary coil which couples with the field when the secondary device is in proximity to the primary unit. A control unit causes a circuit including said primary coil to operate, during a measurement period, in an undriven resonating condition. In this condition the application of the drive signals to the primary coil by the electrical drive unit is suspended so that energy stored in the circuit decays over the course of the period. A decay measurement unit takes one or more measures of such energy decay during the measurement period. In dependence upon said one or more energy decay measures, the control unit controls the electrical drive unit so as to restrict or stop inductive power transfer from the primary unit.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a Continuation of U.S. Ser. No. 15/370,770, filed onDec. 6, 2016, which is a Continuation application of U.S. Ser. No.14/082,979, filed on Nov. 18, 2013, which is a Continuation applicationof U.S. Ser. No. 13/196,298, filed on Aug. 2, 2011, which is aContinuation application of U.S. Ser. No. 12/885,445, filed on Sep. 18,2010, which is a Continuation application of U.S. Ser. No. 12/366,842,filed on Feb. 6, 2009, which is a Division of U.S. Ser. No. 11/569,029,filed Nov. 13, 2006, which claims the benefit of InternationalApplication Serial No. PCT/GB05/01833, filed on May 11, 2005, whichclaims the benefit of United Kingdom Application No. 0410503.7, filed onMay 11, 2004. These applications are hereby incorporated by referenceherein.

The present invention relates to controlling inductive power transfersystems for use, for example, to power portable electrical or electronicdevices.

Inductive power transfer systems suitable for powering portable devicesmay consist of two parts:

-   -   A primary unit having at least one primary coil, through which        it drives an alternating current, creating a time-varying        magnetic flux.    -   A secondary device, separable from the primary unit, containing        a secondary coil. When the secondary coil is placed in proximity        to the time-varying flux created by the primary coil, the        varying flux induces an alternating current in the secondary        coil, and thus power may be transferred inductively from the        primary unit to the secondary device.

Generally, the secondary device supplies the transferred power to anexternal load, and the secondary device may be carried in or by a hostobject which includes the load. For example the host object may be aportable electrical or electronic device having a rechargeable batteryor cell. In this case the load may be a battery charger circuit forcharging the battery or cell.

Alternatively, the secondary device may be incorporated in such arechargeable cell or battery, together with a suitable battery chargercircuit.

A class of such an inductive power transfer systems is described in ourUnited Kingdom patent publication GB-A-2388716. A notable characteristicof this class of systems is the physically “open” nature of the magneticsystem of the primary unit—a significant part of the magnetic path isthrough air. This is necessary in order that the primary unit can supplypower to different shapes and sizes of secondary device, and to multiplesecondary devices simultaneously. Another example of such an “open”system is described in GB-A-2389720.

Such systems may suffer from some problems. A first problem is that theprimary unit cannot be 100% efficient. For example, switching losses inthe electronics and I²R losses in the primary coil dissipate power evenwhen there is no secondary device present, or when no secondary devicesthat are present require charge. This wastes energy. Preferably theprimary unit should enter a low-power “standby mode” in this situation.

A second problem in such systems is that it is not possible tomechanically prevent foreign objects from being placed into proximitywith the primary coil, coupling to the coil. Foreign objects made ofmetal will have eddy-currents induced therein. These eddy currents tendto act to exclude the flux, but because the material has resistance, theflowing eddy currents will suffer I²R losses which will cause heating ofthe object. There are two particular cases where this heating may besignificant:

-   -   If the resistance of any metal is high, for example if it is        impure or thin.    -   If the material is ferromagnetic, for example steel. Such        materials have high permeability, encouraging a high flux        density within the material, causing large eddy currents and        therefore large I2R losses.

In the present application, such foreign objects that cause power drainare termed “parasitic loads”. Preferably the primary unit should enter a“shutdown mode” when parasitic loads are present, to avoid heating them.

Various approaches to solve these two problems have been proposed in theprior art.

Solutions to the first problem, of not wasting power when no secondarydevice requires charge, include:

-   -   In EP0533247 and U.S. Pat. No. 6,118,249 the secondary device        modulates its inductive load during charging, causing a        corresponding variation in the power taken from the primary        unit. This indicates to the primary unit that it should stay out        of the standby state.    -   In EP1022840 the primary unit varies the frequency of its drive,        and thus the coupling factor to a tuned secondary unit. If the        secondary unit is not taking power, there is no difference in        the power taken as the frequency is swept and thus the primary        unit goes into a standby state.    -   In U.S. Pat. No. 5,536,979 the primary unit simply measures the        power flowing in the primary coil, and enters a pulsing standby        state if this is below a threshold.    -   In U.S. Pat. No. 5,896,278 the primary unit contains detecting        coils which have power coupled back into them according to the        position of the secondary device. If the device is not present        the primary unit enters a standby mode.    -   In U.S. Pat. No. 5,952,814 the secondary device has a mechanical        protrusion which fits a slot in the primary unit, activating it.    -   In U.S. Pat. No. 6,028,413 the primary unit drives two coils,        and there are a corresponding two power receiving secondary        coils in the secondary unit. The primary unit measures the power        delivered from each primary coil and enters standby mode if it        is below a threshold.

Solutions to the second problem, of parasitic loads, include:

-   -   As mentioned above, in EP1022840 the primary unit varies the        frequency of its drive. In this system, the secondary device is        tuned, so this frequency variation will result in a variation of        the power taken from the primary unit. If the load is instead a        piece of metal, then varying the frequency will not have as much        effect and the primary unit will enter a shutdown state.    -   As mentioned above, in U.S. Pat. No. 5,952,814 a key in the        secondary device activates the primary unit. The assumption is        that if a secondary device is present then this will physically        exclude any foreign objects.    -   As mentioned above, in U.S. Pat. No. 6,028,413 the primary unit        supplies power to the secondary device by driving two primary        coils. If the amount of power supplied by the two coils is        different, the primary unit assumes that the load is not a valid        secondary device and enters shutdown mode.

These approaches all assume a 1:1 relationship between the primary unitand the secondary device. Therefore they are not sufficient for systemssuch as those described in GB-A-2388716 where more than one secondarydevice at a time may be present. For example, they would not work whenthere are two secondary devices present, one requiring charge and theother not.

Some of these approaches also assume that the physical or electricalpresence of a valid secondary device implies that all foreign objectsare physically excluded by the secondary device. This is not necessarilythe case, particularly when the secondary devices may be positionedarbitrarily in respect of the primary unit, as in those described inGB-A-2388716.

According to a first aspect of the present invention, there is providedan inductive power transfer system comprising a primary unit, having aprimary coil and electrical drive means connected to the primary coilfor applying electrical drive signals thereto so as to generate anelectromagnetic field, and the system also comprising at least onesecondary device, separable from the primary unit and having a secondarycoil adapted to couple with said field when the secondary device is inproximity to the primary unit so that power can be transferredinductively from the primary unit to the secondary device without directelectrical conductive contacts therebetween, wherein the primary unitfurther comprises: control means operable to cause a circuit includingsaid primary coil to operate, during a measurement period, in anundriven resonating condition in which the application of said drivesignals to said primary coil by said electrical drive means is suspendedso that energy stored in said circuit decays over a course of saidperiod; and decay measurement means operable to take one or moremeasures of such energy decay during said period; said control meansbeing further operable, in dependence upon said one or more energy decaymeasures, to control the electrical drive means so as to restrict orstop inductive power transfer from the primary unit.

Such a system is advantageous because it can address either or both thestandby problem and the parasitic load detection problem listed above,in a robust and cost-effective manner, and is particularly advantageousin systems which may have multiple secondary devices present indifferent charge states, and/or whose open magnetic nature makes it easyfor parasitic objects to couple to the primary coil.

In the present application, the term “ring-down” will be used to meancausing the circuit (“resonant tank”) including the primary coil tooperate in this undriven resonating condition.

During a ring-down, no energy is being supplied to the primary coil, andso the decay of energy in the resonant tank is a measure of how muchenergy is being removed from it. The principal causes of energy lossare:

-   -   Energy coupled into the secondary coil of any secondary device        present. This energy may be stored in a storage unit of the        secondary device (if provided) and/or delivered to a load        connected to the secondary device.    -   Losses to any parasitic loads (foreign objects other than valid        secondary devices) present.    -   Other losses in the primary unit or any secondary devices/host        objects present. These other losses include inefficiencies in        the primary coil itself and any other components of the resonant        tank (e.g. I²R losses in the copper of the coil or effective        series resistance of any resonating capacitor). They also        include any magnetic losses in the primary and secondary units,        for example magnetic hysteretic loop losses in any cores        associated with the primary unit and/or secondary device.

In a preferred embodiment the or one energy decay measure is a measureof a rate of such energy decay. In this case, the rate of energy decayis a measure of the rate at which energy is being removed from theresonant tank.

The control means may employ the energy decay measures to detect when asubstantial parasitic load is present in proximity to said primary unit,and restrict or stop inductive power transfer from the primary unitfollowing the detection of such a substantial parasitic load. Forexample, the energy decay rate may be compared with a shutdown 30threshold, and if the rate exceeds the threshold the power transfer isshut down.

Alternatively, or in addition, the control means may employ the energydecay measures to detect when there is no secondary device present inproximity to the primary unit, and restrict or stop inductive powertransfer from the primary unit when no such secondary device isdetected.

When carrying out such detection the control means may employ firstcompensation information relating to a load imposed on the primary unitby losses in the primary unit itself so as to compensate for said lossesof said primary unit. This prevents the detection from being affected bythe inefficiencies in the primary coil itself and any other componentsof the resonant tank, as mentioned above.

The primary unit may further comprise calibration means for derivingpart or all of the first compensation information from measurementstaken by the primary unit when it is effectively in electromagneticisolation.

As noted above, depending on its construction, the secondary device,and/or any host object which carries it, may suffer magnetic lossesintrinsic to its construction, for example in magnetic core material,and other metal used in its construction. These so-called “friendlyparasitics” will be accounted for by the primary unit as a furtherparasitic loss, and if they sum to more than the threshold, the controlmeans will shut the primary unit down. This may be avoided by providinga method of communicating the friendly parasitics of a device to theprimary unit so that they may be accounted for and added to the shutdownthreshold.

Accordingly, when carrying out such detection said control means mayemploy second compensation information relating to a parasitic loadimposed on the primary unit by the secondary device so as to compensatefor the parasitic load of the secondary device.

In this case, preferably the or each secondary device is operable tocommunicate its second compensation information directly to the primaryunit or to communicate to the primary unit other information from whichthe primary unit can derive the second compensation information.

The secondary device may modulate a dummy load to communicate this otherinformation, and the primary unit may derive the second compensationinformation by measuring the dummy load modulation.

This way of communicating the second compensation information also stillworks if multiple secondary devices are present. For example, assumingthe secondary devices apply their dummy loads at the same time, theprimary unit will see a total dummy load equal to the sum of theindividual dummy loads of the secondary devices. This is all that theprimary unit needs to know for compensating for the parasitic loads ofthe secondary devices.

One way of doing this is to have a feedback resistor, whose value isproportional to the value of the friendly parasitics of the secondarydevice, which can be switched across the secondary coil, creating adummy parasitic load. If this feedback resistor is always applied duringcertain ring-downs (for example during the third ring-down of a set),the primary unit can then measure the sum of the incremental loadsresulting from the feedback resistors in each secondary device present(whether its load is drawing power or not) and adjust its thresholdaccordingly. As described below, the same feedback resistor may be usedas a binary on/off signaling means to control standby mode.

Part or all of the first compensation information and/or part or all ofthe second compensation information may be information stored in theprimary unit during manufacture and/or testing of the primary unit.

The system may have information varying means for varying one or both ofthe first and second compensation information when one or more operatingconditions (for example, temperature) of the primary unit vary.

The decay measurement means may take one or more first such energy decaymeasures in a first such measurement period and take one or more secondsuch energy decay measures in a second measurement period. In this casethe secondary device may comprise decay varying means which cause thesecondary device to consume more energy in the first measurement periodthan in the second measurement period. The control means in said primaryunit can then employ the first and second energy decay measures todetect when a secondary device of the system is present in proximity tothe primary unit. For example, if the control means detects asubstantial difference between the first energy measures and the secondenergy measures, it may conclude that a secondary device must bepresent.

The decay varying means may cause the secondary device to impose a firstdummy load on the primary unit in the first measurement period and toimpose a second dummy load, different from the first dummy load, on saidprimary unit in the second measurement period. One of the first andsecond dummy loads may be zero.

In this way, the secondary device may apply an additional dummy (or“feedback”) load only during some ring-downs and not during others, oronly during some part of the ring-down but not others. This may be donefor example by switching the feedback resistor mentioned above acrossthe secondary coil, creating a dummy parasitic load, as measured by theprimary unit, which varies with time. In contrast, true parasitic loads(for example a piece of steel) will appear to be a constant parasiticload. If the primary unit control means takes similar load measurementsbetween different ring-downs, or different parts of the same ring-down,it may put the drive means into a power-saving “standby” mode. This modewill be entered when no valid secondary device is present.

The decay varying means may be disabled when an actual load of thesecondary device requires no power. If the secondary device onlypresents a time-varying load during ring-down when it requires power,then the primary unit will enter a standby mode even when there aresecondary devices present, if they don't require power.

A difference between the first dummy load and the second dummy load maybe set in dependence upon said parasitic load imposed on said primaryunit by said secondary device. This is a convenient way to communicatethe second compensation information to the primary unit.

The secondary device may be adapted to supply the power it receivesinductively from the primary unit to an actual load external to thesecondary device. The load may be physically separable from the rest ofthe secondary device, for example it may be connected by a removableplug and socket arrangement. Of course, the secondary device willgenerally include its own circuitry which will need some of the receivedpower too. In some cases the load could be internal to the secondarydevice.

The secondary device preferably comprises a storage means for storingenergy received from the primary unit. The storage means, whereprovided, supplies stored energy to the actual load and/or circuitry ofthe secondary device when the circuit is operating in the undrivenresonating condition. This is not essential in all embodiments. Any loadwhich can cope with short power reductions or interruptions will notrequire the storage means.

The secondary device may comprise load isolation means operable duringthe measurement period to prevent the supply to an actual load of thesecondary device of any of the power being received inductively by thesecondary device from the primary unit. Ideally during a ring-down thestorage means of the secondary device is of sufficient capacity, andsufficiently well-charged before the ring-down commences, that the decayof energy in the storage means happens more slowly than the decay ofenergy in the primary coil. If this cannot be guaranteed to be the caseunder all conditions, for example if the load is separable and withunknown characteristics, then the load isolation means can be includedin the secondary device to disconnect the secondary device's storagemeans, and load, during the ring down, to ensure that its electricalload is not measured by the primary unit.

When the inductive power transfer from the primary unit has been set tosuch a restricted or stopped state, for example because the parasiticload became too high, the control means may cause the circuit to operatein the undriven resonating condition during a series of intermittentprobing periods. In this case the decay measurement means takes one ormore such energy decay measures during each said probing period. Thecontrol means employ the energy decay measures taken during the probingperiods to determine when to end the restricted or stopped state. Forexample, the control means may periodically allow the drive to run for ashort while in order to conduct a ring-down to measure the parasiticload and, if it is now below a threshold, resume the drive.

Alternatively, when the inductive power transfer from the primary unithas been set into such a restricted or stopped state, the control meansmay maintain that state until the primary unit is reset by a user of theprimary unit.

It may be advantageous to provide a clear means of synchronising thebehaviour of the secondary device with that of the primary unit. Forexample, if the secondary device must isolate the load during ring-down,it may need a clear signal that such a ring-down is about to happen,since opening it once a ring-down is in process may be too late. Thereare many possible means of such synchronisation using a variety ofcommunications means, but one convenient and reliable method is to usethe power transfer channel between primary unit and secondary device byputting onto the primary coil a signal which does not occur duringnormal operation. For example, the primary unit may change theamplitude, frequency or phase of its drive.

The primary unit may comprise synchronising means operable to transmit apredetermined synchronising signal to the secondary device tosynchronise operation of the secondary device with that of the primaryunit.

The synchronising means may send the predetermined synchronising signalprior to causing the circuit to operate in said undriven resonatingcondition.

The primary unit may further comprise snubber means. In this case, thesynchronizing means may switch the snubber means into the circuit aftersuspending the application of electrical drive signals to said primarycoil, so as to send the synchronising signal to the secondary device. Inthis way the primary coil is stopped very quickly by switching in thesnubber, which provides a very easily detectable synchronising signal.

The storage means (e.g. capacitor) of the secondary device may be ofsignificant size and cost if it is to provide sufficient capacity tosupply the load during a ring-down. Therefore it is advantageous for thering-down to occur as quickly as possible. Measuring fewer cycles of theprimary coil is one way to achieve this. This will reduce the accuracyof the system but this may be compensated for by averaging the resultsof several ring-downs.

The primary unit may further comprise resonant frequency increasingmeans for temporarily increasing a resonant frequency of the circuitduring operation thereof in the undriven resonating condition. This isanother way to shorten the ring-down time, because by increasing theresonant frequency of the primary coil during a ring-down, more cyclesoccur in less time.

The control means preferably cause the electrical drive means to resumeapplication of the drive signals to the primary coil in phase with anyresidual resonating energy within the circuit. After a ring-down iscomplete, the primary unit must resume normal operation as quickly aspossible to prevent the secondary device storage means from dischargingto the point where the supply to the load becomes inadequate. Restartingthe drive to the primary coil in-phase with any residual resonatingenergy within the primary coil can ensure that the coil returns tonominal operating conditions as quickly as possible.

The control means may cause the electrical drive means to temporarilyboost the electrical drive signals prior to causing the circuit tooperate in the undriven resonating condition. This can enhance thehold-up of the secondary device storage means, for any given such means,keeping the size and cost of such means to a minimum. Before aring-down, the control means may cause a boost in the current in theprimary coil for one or more cycles, thus temporarily raising thevoltage in the secondary device storage means. This stores more energyin that means, enabling it to power the load for longer duringring-down.

The control means may measure a natural resonant frequency of thecircuit during the measurement period and employ the measured frequencyto compensate for an influence of changes in the natural resonantfrequency on the energy decay measures. Because the resonant tank is notbeing actively driven during ring-down, the primary coil resonates atthe natural resonant frequency of the system. Measuring the naturalresonant frequency during ring-down allows a calculation of theinductance to be made, leading to a direct calculation of total powerloss. The inductance may change due to the presence of secondary devicesand/or parasitic loads.

The control means may cause the circuit to operate in the undrivenresonating condition during a series of different such measurementperiods. In this case, the decay measurement means may take one or moresuch energy decay measures in each said measurement period. The controlmeans can then employ the energy decay measures taken in two or moredifferent measurement periods to control said electrical drive means.For example, the control means can employ an average of the energy decaymeasures taken in different measurement periods.

Two or more successive measurement periods may be part of the samering-down. This can give fast results. Alternatively, there may be justone measurement period in each ring-down. This gives slower results butit is possible to resume application of drive signals to the primarycoil between measurement periods, making the power interruption to thesecondary devices shorter. Also, the circuit conditions can bereplicated at the start of each measurement period. The duration of eachring-down is preferably short compared to an interval between successivering-downs.

In some systems, the primary coil may not driven continuously. It may,for example, be driven in pulses, for example in a pulse-width-modulatedmanner. In such systems, a partial “ring-down” is effectively happeningwhenever the primary coil is not being actively driven, perhaps once ormore per cycle. Measurement of the natural resonant frequency and decayof the primary coil during these un-driven periods can provide the samefunctionality as that from the multiple-cycle ring-downs describedabove.

The secondary device may have a timing means capable of changing thebehaviour of the secondary device over time. For example, the timingmeans may detect when a second ring-down has happened shortly after afirst ring-down, and, for example, switch in a conditional dummy(feedback) load only on such a shortly-following second ring-down. Thismay be useful for example in making interactions between the primaryunit and secondary device more deterministic. For example such a meanscould be used to make the standby detection scheme described above morereliable. For example, if the primary unit always conducts ring-downs inpairs a few milliseconds apart but repeated once every second, and thesecondary device applies the feedback load only on the second of the tworing-downs of a pair, the primary unit may average several “first ringdowns”, and separately average several “second ring-downs”, to improvethe accuracy of measurement.

The invention can operate even in systems which have more than oneprimary coil. An example is systems which allow complete freedom ofposition or orientation of the secondary device by having two orthogonalcoils, driven in quadrature, as in GB-A-2388716. In such systems therewill be some coupling between the two primary coils, which may make itawkward to analyse a “ring-down” occurring simultaneously on both coils.In this case, the coils can be tested alternately. At the start of atest, both coils are halted, but then only the first coil is allowed toring-down while the second coil is held in a halted state to prevent itfrom affecting the measurements on the first coil. During the next testthe first coil is held halted while the second coil is allowed toring-down.

Accordingly, in a preferred embodiment the primary unit may have firstand second such primary coils. A first such electrical drive means isconnected to the first primary coil for applying electrical drivesignals thereto so as to generate a first electromagnetic field, and asecond such electrical drive means is connected to the second primarycoil for applying electrical drive signals thereto so as to generate asecond electromagnetic field. The control means causes the secondelectrical drive means to suspend application of electrical drivesignals to the second primary coil whilst a first such circuit includingthe first primary coil is in the undriven resonating condition and thedecay measurement means are taking one or more such measures of energydecay in the first circuit. Similarly the control means causes the firstelectrical drive means to suspend application of electrical drivesignals to the first primary coil whilst a second such circuit includingsaid second primary coil is in the undriven resonating condition and thedecay measurement means are taking one or more such measures of energydecay in the second circuit.

The operating conditions of the primary unit after the ring-down iscomplete are likely to be identical to those before the ring-downstarted. If the primary unit contains any control loops (for example tomaintain the strength of the magnetic flux), these control loops may befrozen during the ring-down, so that they do not respond to thering-down and can 20 resume normal operation immediately.

At least one secondary device may be carried in or by an objectrequiring power. The secondary device can receive power inductively fromthe primary unit when the object carrying the secondary device is placedon or in proximity to the primary unit. The object may be a portableelectrical or electronic device, for example a mobile communicationdevice.

The system may have an “open” construction with a power transfersurface, the field generated by said primary unit being distributed overa power transfer area of the surface such that, when such a secondarydevice is placed on or in proximity to the power transfer area, thedevice can receive power inductively from the primary unit.

The power transfer area may be large enough that two or more suchsecondary devices can be placed on or in proximity to the power transferarea to receive power simultaneously from the primary unit.

According to a second aspect of the present invention there is provideda primary unit, for use in an inductive power transfer system, theprimary unit comprising: a primary coil; electrical drive meansconnected to said primary coil for applying electrical drive signalsthereto so as to generate an electromagnetic field; control meansoperable to cause a circuit including said primary coil to operate,during a measurement period, in an undriven resonating condition inwhich the application of said drive signals to said primary coil by saidelectrical drive means is suspended so that energy stored in saidcircuit decays over the course of said period; and decay measurementmeans operable to take one or more measures of such energy decay duringsaid period; said control means being further operable, in dependenceupon said one or more energy decay measures, to control the electricaldrive means so as to restrict or stop inductive power transfer from theprimary unit.

According to a third aspect of the present invention there is provided asecondary device, for use in an inductive power transfer system thatcomprises a primary unit which generates an electromagnetic field, thesecondary device comprising: a secondary coil adapted to couple withsaid field generated by said primary unit when the secondary device isin proximity to the primary unit so that power can be receivedinductively by the secondary device from the primary unit without directelectrical conductive contacts therebetween; load connection means,connected to said secondary coil and adapted to be connected when thesecondary device is in use to a load requiring power from the primaryunit, for supplying such inductively-received power to the load; andcommunication means operable to communicate to the primary unitinformation relating to a parasitic load imposed on the primary unit bythe secondary device.

The communication means may communicate the information by imposing adummy load on said primary unit.

The communication means may impose a first dummy load on the primaryunit at a first time and a second dummy load, different from the firstdummy load, at a second time, a difference between the first and seconddummy loads being set in dependence upon the parasitic load. One of thefirst and second dummy loads may be zero.

This way of communicating the information still works if multiplesecondary devices are present. For example, assuming the secondarydevices apply their dummy loads at the same time, the primary unit willsee a total load equal to the sum of the individual dummy loads of thesecondary devices. In this way, the total parasitic load from multiplesecondary devices can be communicated very efficiently and reliably.

Alternatively, the communication means may set a time period in which adummy load is imposed on the primary unit in dependence upon theparasitic load imposed on the primary unit by the secondary device. Theprimary unit can then measure the duration of the period to obtaininformation relating to the parasitic load.

According to a fourth aspect of the present invention there is provideda portable electrical or electronic device comprising: a load which atleast at times requires power from said primary unit; and a secondarydevice embodying the aforesaid third aspect of the present invention,said load connection means of said secondary device being connected tosaid load for supplying such inductively-received power to the load atsaid times.

According to a fifth aspect of the present invention there is provided amethod of controlling inductive power transfer in an inductive powertransfer system comprising a primary unit, having a primary coil towhich electrical drive signals are applied to 30 generate anelectromagnetic field, and also comprising at least one secondarydevice, separable from the primary unit and having a secondary coiladapted to couple with said field when the secondary device is inproximity to the primary unit so that power can be transferredinductively from the primary unit to the secondary device without directelectrical conductive contacts therebetween, which method comprises:causing a circuit including said primary coil to operate, during ameasurement period, in an undriven resonating condition in which theapplication of said drive signals to said primary coil is suspended sothat energy stored in the circuit decays over the course of said period;taking one or more measures of such energy decay during said period; andrestricting or stopping inductive power transfer from the primary unitin dependence upon said one or more energy decay measures.

Reference will now be made, by way of example, to the accompanyingdrawings in which:

FIG. 1 is a block diagram showing parts of an inductive power transfersystem embodying the present invention;

FIGS. 2(A) and (B) show example waveforms generated in the FIG. 1system;

FIG. 3 is a graph illustrating how energy decays in different parts ofthe system of FIG. 1;

FIG. 4 is a block diagram showing parts of an inductive power transfersystem according to a preferred embodiment of the present invention;

FIG. 5 shows different operating states of a circuit in the FIG. 4system when the circuit is in an undriven resonating condition;

FIG. 6 is a graph illustrating a variation over time of current flowingthrough a primary coil in each of the FIG. 5 states;

FIG. 7 is a diagram illustrating waveforms generated in operation of theFIG. 4 system;

FIG. 8 is a diagram for illustrating different modes of operation in theFIG. 4 system; and

FIGS. 9(A) to (E) illustrate the conditions under which the modes ofFIG. 8 are selected in the FIG. 4 system.

FIG. 1 illustrates parts of an inductive power transfer system embodyingthe present invention. The system 1 comprises a primary unit 10 and atleast one secondary device 30. The primary unit 10 has a primary coil 12and an electrical drive unit 14 connected to the primary coil 12 forapplying electrical drive signals thereto so as to generate anelectromagnetic field.

The primary unit may have any suitable form but one preferred form is aflat platform having a power transfer surface on or in proximity towhich each secondary device can be placed. In this case, the field maybe distributed over a power transfer area of the surface, as describedin GB-A-2388716.

The secondary device 30 is separable from the primary unit 10 and has asecondary coil 32 which couples with the electromagnetic field generatedby the primary unit when the secondary device is in proximity to theprimary unit. In this way, power can be transferred inductively from theprimary unit to the secondary device without direct electricalconductive contacts therebetween.

The primary and secondary coils can have any suitable forms, but may forexample be copper wire wound around a high-permeability former such asferrite or amorphous 20 metal.

The secondary device is usually connected to an external load (notshown) and supplies the inductively-received power to the external load.The secondary device may be carried in or by an object requiring powersuch as a portable electrical or electronic device or a rechargeablebattery or cell. Further information regarding designs of secondarydevice and the objects which can be powered using the secondary devicescan be found in GB-A-2388716.

The primary unit 10 in the FIG. 1 system also comprises a control unit16 and a decay measurement unit 18. The control unit causes a circuit(referred to hereinafter as a “resonant tank”) including the primarycoil to operate in an undriven resonating condition during a measurementperiod. In this undriven resonating condition (“ring-down”) theapplication of electrical drive signals to the primary coil by theelectrical drive unit 14 is suspended so that energy stored in theresonant tank decays over the course of the measurement period.

Typically a resonant tank is formed by a circuit comprising at least aninductor and a capacitor. The resonant tank stores electrical energy.When brought into the undriven resonating condition, the storedelectrical energy flows or oscillates within the tank at a naturalresonant frequency of the tank.

The resonant tank includes not only the electrical circuit within theprimary unit of which the primary coil is a part, but also theelectrical circuit(s) within the at least one secondary units, which isor are coupled to the primary coil via the magnetic circuit formed bythe primary and secondary coil(s).

The primary coil may be made resonant with a capacitor in series orparallel, or both series and parallel, and/or may be resonant due to itsself-capacitance. It may or may not be driven at its resonant frequencyin normal operation, but once a ring-down occurs will naturallyoscillate at its resonant frequency.

During a ring-down, no energy is being supplied to the primary coil, andso the rate of decay of energy in the resonant tank is a measure of therate at which energy is being removed from it. The principal causes ofenergy loss are:

-   -   Energy coupled into the secondary coil of any secondary device        present. This energy may be stored in a storage unit of the        secondary device and/or delivered to a load connected to the        secondary device.    -   Losses to any parasitic loads (foreign objects) present.    -   Other losses in the primary unit or any secondary devices/host        objects present. These other losses include inefficiencies in        the primary coil itself and any other components of the resonant        tank (e.g. I²R losses in the copper of the coil or effective        series resistance of any resonating capacitor). They also        include any magnetic losses in the primary and secondary units,        for example magnetic hysteretic loop losses in any cores        associated with the primary unit and/or secondary device.

FIGS. 2(A) and 2(B) show two possible examples of energy decay during aring-down. If the losses are small (FIG. 2(A)) then during a ring-downthe energy in the resonant tank will decay slowly. Conversely if thelosses are large (FIG. 2(B)) then during a ring-down the energy in theresonant tank will decay rapidly.

The decay measurement unit 18 takes one or more measures of the energydecay in the resonant tank during the measurement period.

The decay measurement unit 18 may take the energy decay measures in anysuitable way, for example by measuring the voltage across, or currentthrough, the primary coil or by measuring some other quantity associatedwith the primary coil, for example the magnetic flux generated thereby.The measurement may be taken over part of a cycle, or over one or morecycles. The measure may be, for example, the ratio of one peak to asubsequent peak. Alternatively, the measure may be derived bycurve-fitting several data points. The measure is a rate of energy decayin the resonant tank in one preferred embodiment.

The control unit 16 controls the electrical drive unit 14 so as torestrict or stop inductive power transfer from the primary unit independence upon the energy decay measures produced by the decaymeasurement unit 18.

The energy decay measures may be used by the control unit to restrict orstop inductive power transfer from the primary unit following thedetection of certain conditions.

One such condition is the presence of a substantial parasitic load inthe vicinity of the primary unit. In this case, the control unit 16 mayenter a shutdown mode in which the drive to the primary coil is reducedor stopped, preventing heating of the parasitic load.

Another such condition is when no secondary device of the system ispresent in the vicinity of the primary unit. Another such condition iswhen there is at least one secondary device present but none of thedevices has a load currently requiring power. A load does not requirepower, for example, when turned off or when, in the case of arechargeable battery or cell, the battery or cell is fully charged.Under both these conditions the control unit 16 may enter a standby modein which the drive to the primary coil is reduced or stopped, preventingunnecessary power consumption in the primary unit.

Compared to other possible methods of measuring the power delivered tothe primary coil—for example by measuring the supply power to theprimary unit drive electronics—the method used in the present inventionis quick and accurate. This accuracy is due at least to the followingaspects:

-   -   The drive electronics may be inactive and largely out of circuit        during the ring-down, therefore their noise and tolerances do        not affect the measurement accuracy.    -   Since the method directly measures power taken, it is very hard        to “trick”—it works reliably regardless of how power is being        taken out, in contrast to other indirect techniques such as        measuring inductance shift which make assumptions between the        quality they measure and the parasitic load, which may not        always be true.

FIG. 4 shows parts of an inductive power transfer system according to apreferred embodiment of the present invention. The system 1 has aprimary unit 10 and a secondary device 30. FIG. 4 also shows a parasiticload 500 on the primary unit, caused for example by a foreign objectplaced in the vicinity of the primary unit. The secondary device 30 inthis case is assumed to be carried in or by a host object such as aportable electrical or electronic device. As explained hereinbefore thesecondary device and/or host object also inevitably impose a “friendly”parasitic load 501 on the primary unit.

As described earlier with reference to FIG. 1, the primary unit 10comprises a primary coil 12, an electrical drive unit 14, a control unit16 and a decay measurement unit 18.

The electrical drive unit 14 in this embodiment has a conventional halfbridge configuration in which a first switch 20 is connected between afirst power supply line of the primary unit and an output node of theelectrical drive unit, and a second switch 21 is connected between theoutput node and a second power supply line of the primary unit. Thefirst and second switches 20 and 21 may, for example, be field-effecttransistors (FETs). The electrical drive unit 14 also comprises a drivecontroller 19 which applies control signals to the switches 21 and 22 toturn them on and off. The drive controller 19 has a control inputconnected to an output of the control unit 16

The output node of the electrical drive unit 14 is connected via acapacitor 17 to one side of the primary coil 12.

The control unit 16 is a microprocessor in this embodiment.Alternatively, an ASIC could be used to implement the control unit 16,as well as some or all of the other circuit elements of the primaryunit.

The decay measurement unit 18 in this embodiment comprises a resistor 25which has a first node connected to one side of a switch 28 and a secondnode connected to the second power supply line. The resistor 25 is alow-value resistor. The decay measurement unit 18 further comprises anoperational amplifier 26 having an input connected to the first node ofthe resistor 25. The decay measurement unit 18 also comprises ananalog-to-digital converter (ADC) 27 connected to an output of theoperational amplifier 26. An output of the ADC 27 is connected to ameasurement input of the control unit 16.

The other side of the switch 28 is connected to the other side of theprimary coil 12. A snubber unit 22 is connected in parallel with theswitch 28. The snubber unit 22 comprises a capacitor 23 and a resistor24 connected in series with one another.

The primary unit 10 further comprises a calibration unit 29 in thisembodiment. The calibration unit 29 stores first compensationinformation about the losses in the primary unit (e.g. electrical ormagnetic losses). By design, at manufacture, and/or periodicallythereafter, the losses in the primary unit may be calibrated and storedwithin the calibration unit 29. The calibration unit supplies the storedinformation to the control unit 16 to enable the control unit 16 tosubtract the losses from the total measurement, thus calculating anumber for the loss due to parasitic loads alone. This calibration unit29 may vary the first compensation information to cope with variablelosses in the primary unit, for example losses which vary withtemperature.

The secondary device 30 comprises a secondary coil 32, a rectifier 34, asecondary control unit 36, a dummy load switch 38, a dummy load 40, aload switch 42, a storage unit 44 and an actual load 46. The dummy loadswitch 38 and the load switch 42 may each be an FET, for example. Thedummy load 40 is, for example, a resistor. The storage unit 44 is acapacitor in this embodiment but an inductor could be used instead. Theactual load 46 is external to the secondary device in this embodimentand is part of the host object. It could be a battery charge controllerfor a Lithium-ion cell.

Operation of the system of FIG. 4 will now be described.

In an “operating mode” of the system, the host object incorporating thesecondary device 30 is placed on or in proximity to the primary unit 10.The drive controller 19 applies control signals to the switches 20 and21 alternately, so that electrical drive signals are applied to theprimary coil 12 via the capacitor 17. The switch 28 is closed. Theprimary coil 12, capacitor 17, switch 28 and resistor 25 and parts ofthe secondary device 30 together form a resonant tank. The resonant tankis preferably driven by the electrical drive unit 14 at or nearresonance so as to maximise power transfer to the secondary device, butthis is not essential in the present invention. In its driven condition,the resonant tank could be driven at some frequency other than itsresonant frequency.

In the operating mode, the primary coil 12 generates an electromagneticfield in the vicinity of the primary unit. The secondary coil 32 coupleswith this field and an alternating current is induced in the coil by thefield. The dummy load switch 38 is open and the load switch 42 isclosed. The alternating current induced in the secondary coil 32 isrectified by the rectifier 34 and the rectified current is supplied viathe load switch 42 to the storage unit 44 and the actual load 46. Inthis way, power is transferred inductively from the primary unit to thesecondary device 30 and from there to the load. The storage unit 44stores energy in the operating mode.

Whilst in the operating mode, from time to time the control unit 16 inthe primary unit initiates a series of three ring-downs in quicksuccession. Such a series of three ring-downs is shown in FIG. 7. Theinterval between successive series of ring-downs may be, for example,one second. The duration of each ring-down is, for example, onemillisecond. Each ring-down of a series has a sequence of three statesas shown in FIG. 5.

Initially, the system has a normal state in which the control unit 16causes the electrical drive unit 14 to apply drive signals to theprimary coil 12 to cause it to oscillate. It will be appreciated that inthe operating mode as described above, the system is in this state foralmost all the time.

The next state is a “snub” state. The application of drive signals tothe primary coil 12 by the electrical drive unit 14 is suspended underthe control of the control unit 16. The drive controller 19 closes theswitch 21. The control unit 16 also opens the switch 28 at a time whenmost of the energy in the resonant tank resides in the capacitor 17. Theopening of the switch 28 brings the snubber unit 22 in series with theresonant tank. The snubber unit 22 quickly dissipates any energy whichremains in the primary coil 12, stopping it from resonating within onecycle or so. Most of the energy stored in the resonant tank is left inthe capacitor 17.

The sudden cessation of cycles is detected by the secondary control unit36 in the secondary device 30. The secondary control unit 36 opens theload switch 42. The system then enters the decay state from the snubstate.

The control unit 16 closes the switch 28, removing the snubber unit 22from the resonant tank, and thus allowing the energy in capacitor 17 toflow again within the resonant tank. In the decay state, the resonanttank operates in an undriven resonating condition. Energy stored in theresonant tank decays over the course of time in the decay state. In thisembodiment, the decay measurement unit 18 measures the energy decay inthe resonant tank by measuring the current flowing through the primarycoil 12. The same current flows through the resistor 25 and generates avoltage at the first node of that resistor. This voltage is buffered bythe operational amplifier 26 and converted into a digital signal by theADC 27. The resulting digital signal is applied to the measurement inputof the control unit 16.

The secondary device uses the storage unit 44 to store energy from theprimary unit during normal operation. During the decay state, the energystored in the storage unit 44 of the secondary device gradually decaysas energy is delivered to the load. Provided that the storage unit hassufficient capacity, and is sufficiently well-charged before ring-downcommences, the storage unit can deliver continuous power to thesecondary device load throughout the ring-down, so the actual load 46 isnot interrupted. In FIG. 3 a solid line 411 shows the decay of energy inthe primary coil 12 and a dotted line 410 represents the decay of energyin the storage unit 44 of the secondary device during the decay state.

The secondary device load will not take any power from the primary coil,and will thus be “invisible” to the primary unit, during the decaystate. This is possible in this embodiment because the rectifier 34between the secondary coil and the storage unit allows current to flowonly from the former to the latter. It can be arranged in other waysthat the secondary device load will not take any power from the primarycoil during the ring-down.

As long as this is the case, the decay measurement will not includelosses due to power supplied to the secondary load, i.e. will onlymeasure losses in the primary unit and losses due to parasitic loads.

FIG. 6 shows how the current flowing through the primary coil 12 variesin the normal, snub and decay states which occur during a ring-down. Inthis embodiment, the digital signals representing the current flowing inthe primary coil within a measurement period are received and processedwithin the control unit 16 to calculate a measure of the rate of energydecay in the resonant tank.

An equation describing the energy stored in the resonant tank, atresonance, is:

$E = {{\frac{1}{2}L{\hat{I}}^{2}} = {\frac{1}{2}C{\hat{V}}^{2}}}$

where E is the energy, L the inductance, Î is the peak current, C is thecapacitance and {circumflex over (V)} is the peak voltage.

Therefore the energy stored in the resonant tank of the primary at anygiven moment can be calculated if the inductance and peak current areknown, or if the capacitance and peak voltage are known, or combinationsthereof. Typically the capacitance is known by design, the peak currentand voltage can be measured by suitable circuitry, and the inductancecan be deduced by observing the natural resonant frequency duringring-down and applying the formula:

$L = \frac{1}{4\pi^{2}f^{2}C}$

The rate of decay of energy (and thus the loss) from the resonant tankcan be calculated by measuring E₁ at time T₁ and E₂ at another time T₂,and calculating

$\frac{E_{2} - E_{1}}{T_{2} - T_{1}}$

Since at resonance the voltage and current in the resonant tank will be90 degrees out of phase with one another, a convenient method of readingthe peak voltage of one is to trigger the measurement on thezero-crossing of the other.

The control unit 16 employs the first compensation information stored inthe calibration unit 29 when processing the digital signals to measurethe energy decay rate. In this way, the control unit 16 compensates forthe losses arising in the primary unit itself.

The behaviour of the primary unit and the secondary device is slightlydifferent in each of the three ring-downs of a series.

During the first ring-down, the secondary control unit 36 has the dummyload switch 38 open so that the dummy load 40 is not connected to thesecondary coil 32. As a result, the energy decay measure produced duringthe first ring-down is a measure of the energy decay due to anyparasitic loads 500 from foreign objects in the vicinity of the primaryunit and any parasitic load 501 imposed by losses in the secondarydevice and/or its host object.

During the second ring-down, the secondary control unit 36 selectivelycloses the dummy load switch 38. The secondary control unit 36 decideswhether to have the dummy load switch 38 open or closed during thesecond ring-down based on the power requirement of the actual load 46.If the load 46 does not require any power at the present time, forexample, because it has a rechargeable battery which is presently fullycharged, then the dummy load switch 38 is kept open during the secondring-down. If, on the other hand, the load 46 does require power at thepresent time, then the dummy load switch 38 is closed so that the dummyload 40 is connected to the primary coil 32.

The control unit 16 produces another measure of the energy decay rateduring a measurement period within the second ring-down. If the energydecay rate during the second ring-down is substantially different fromthe energy decay rate during the first ring-down, the control unit 16detects that a secondary device requiring power may be present in thevicinity of the primary unit.

During the third ring-down, the secondary control unit 36 always closesthe dummy load switch 38 so that the dummy load 40 is connected to thesecondary coil 32. Another measure of the energy decay rate is taken bythe control unit 16 in the primary unit. In this case, the energy decayrate is a measure of the sum of the parasitic loads 500, the parasiticload 501 of the secondary device and/or host object, and the dummy load40. Based on the difference between the energy decay rates in the firstand third ring-downs, the control unit calculates the value of the totalof the dummy loads 40 in all of the secondary devices present in thevicinity of the primary unit.

Each dummy load 40 in a secondary device 30 in the system of thisembodiment is set to a particular value (at manufacture or duringcalibration or testing) so that the value represents the parasitic load501 imposed by the secondary device concerned and/or by its host object.

Thus, the total dummy load for all secondary devices present, ascalculated by the control unit 16, can be used by the control unit 16 assecond compensation information to compensate for the parasitic loads501 of the secondary devices present. For example, if the control unit16 detects that a substantial parasitic load 500 is present in thevicinity of the primary unit when the energy decay rate exceeds somethreshold, the threshold may be increased by an amount dependent on thetotal parasitic load 501 of all the secondary devices present, so thatthe detection of parasitic loads 500 from foreign objects is notinfluenced by the number of secondary devices present.

A system embodying the present invention is capable of measuring loadsimposed on the primary unit sensitively, for example to within 50 mW orso. With this degree of sensitivity, it is possible to ensure that verylittle power is coupled into parasitic loads 500 such as foreignobjects.

FIG. 8 is a diagram illustrating the different modes of operation in theFIG. 4 system and the conditions for switching between these differentmodes. The three modes of operation are an operating mode, a shutdownmode and a standby mode.

In the operating mode, the primary unit is in the normal state (drivencondition) most of the time, but periodically does a series of threering-downs as described above. If the result of the ring-downs is thatno secondary device requires power, the primary unit goes into a standbymode. If the result of the ring-downs is that a significant parasiticload 500 is present, the primary unit goes into a shutdown mode.

In the standby mode, the electrical drive unit 14 is stopped for most ofthe time, thus consuming little power. Periodically the primary unitenters the normal state then does a series of ring-downs in respectiveprobing periods, to check whether it should enter either the operatingmode or the shutdown mode. Otherwise it remains in the standby mode.

The shutdown mode is functionally identical to standby mode. However,the two modes may be distinguished by some user-interface feature suchas an LED to prompt the user to remove any substantial parasitic load500.

FIGS. 9(A) to 9(E) illustrate the conditions under which the modes ofFIG. 8 are selected in the FIG. 4 system. In FIG. 9(A) there is nosecondary device present in the vicinity of the primary unit. In thiscase, the primary unit is in the standby mode. In FIG. 9(B) no secondarydevice is present but a substantial parasitic load is present in thevicinity of the primary unit. In this case, the primary unit is in theshutdown mode. In FIG. 9(C) a secondary device and a substantialparasitic load are both present at the same time in the vicinity of theprimary unit. In this case, the primary unit is in the 30 shutdown mode.In FIG. 9(D) a secondary device is present in the vicinity of theprimary unit, but the load connected to the secondary device does notneed any power at the current time. In this case, the primary unit is inthe standby mode. Finally, in FIG. 9(E) a secondary device is presentand its load needs power to charge or operate. Thus, the primary unit isin the operating mode.

1. A wireless power transmitter comprising; electrical drive circuitry;a coil coupled to the electrical drive circuitry and configured togenerate an electromagnetic field, wherein the electromagnetic field isconfigured to inductively transmit wireless power; control circuitrycoupled to the coil and configured to measure the effect of a loadassociated with a wireless power receiver on one or more of the currentthrough or the voltage across the coil, wherein the control circuitry isconfigured to derive, based on the measurement, information related tothe load imposed by the wireless power receiver.
 2. The wireless powertransmitter as claimed in claim 1, wherein the information includesinformation regarding a parasitic load of the wireless power receiver.3. The wireless power transmitter as claimed in claim 2, wherein thecontrol circuitry is configured to measure a difference between thecurrent through the coil at a first time instant and second time instantto derive the information.
 4. The wireless power transmitter as claimedin claim 2, wherein the control circuitry is configured to measure adifference between the voltage across the coil at a first time instantand a second time instant to derive the information.
 5. The wirelesspower transmitter as claimed in claim 1, wherein the control circuitryis configured to detect a first feedback load at a first time instantand a second feedback load at a second time instant, and wherein thefirst and second feedback loads are set in dependence upon a parasiticload of the wireless power receiver.
 6. The wireless power transmitterof claim 5, wherein one of said first and second feedback loads is zero.7. A wireless power transmitter comprising; electrical drive circuitry;a coil coupled to the electrical drive circuitry and configured togenerate an electromagnetic field, wherein the electromagnetic field isconfigured to inductively transmit wireless power; control circuitrycoupled to the coil and configured to: derive information indicative ofthe friendly parasitics of a wireless power receiver based on ameasurement of one or more of the current through or voltage across thecoil; detect in the primary unit whether a substantial parasitic load ispresent in proximity to the primary unit based on the informationindicative of the friendly parasitics of the wireless power receiver;restricting inductive power transfer from the wireless power transmitterbased on the detection.
 8. The wireless power transmitter as claimed inclaim 7, wherein the control circuitry is configured to measure adifference between the current through the coil at a first time instantand second time instant to derive the information.
 9. The wireless powertransmitter as claimed in claim 7, wherein the control circuitry isconfigured to measure a difference between the voltage across the cod ata first time instant and a second time instant to derive theinformation.
 10. The wireless power transmitter as claimed in claim 7,wherein the control circuitry is configured to detect a first feedbackload at a first time instant and a second feedback load at a second timeinstant, and wherein the first and second feedback loads are set independence upon a parasitic load of the wireless power receiver.
 11. Thewireless power transmitter of claim 10, wherein one of said first andsecond feedback loads is zero.
 12. A wireless power receiver comprising;a coil configured to inductively receive power via an electromagneticfield; a load connection circuit, connected to said coil, and configuredto be connected when the wireless power receiver is in use to a loadrequiring power to supply the inductively-received power to the load;and a communication circuit operable to communicate to a wireless powertransmitter information relating to a parasitic load of the wirelesspower receiver, wherein said communication circuit is configured tocommunicate said information by imposing a feedback load on the wirelesspower transmitter.
 13. The wireless power receiver as claimed in claim12, wherein said communication circuit is configured to impose a firstfeedback load on the wireless power transmitter at a first time instantand a second feedback load, different from said first feedback load, ata second time instant, and wherein a difference between said first andsecond feedback loads being based on the parasitic load.
 14. Thewireless power receiver as claimed in claim 13, wherein one of saidfirst and second feedback loads is zero.
 15. The wireless power receiveras claimed in claim 12, wherein said feedback load has a value selectedto communicate a friendly parasitic load of the wireless power receiver.16. A wireless power receiver as claimed in claim 12, wherein saidfeedback load is a resistor having a resistance value selected as aproportion of a friendly parasitic load of the wireless power receiver.17. A wireless power receiver as claimed in claim 12, wherein saidfeedback load is a feedback resistor having a resistance value selectedas a function of a friendly parasitic load of the wireless powerreceiver; and wherein said communication circuit includes a switch forselectively switching said feedback resistor across the coil.